It is a common technology to use Mach-Zehnder interferometer (MZI) to construct an optical waveguide switch or modulator. In U.S. Pat. No. 7,817,881, the waveguide capacitor concept is introduced in which the free carriers can be stored inside the waveguide core in order to modulate the refractive index of the material.
In silicon photonics, the free-carrier dispersion effect is used to modulate the refractive index of the silicon, and then, produces a switch or amplitude modulation through the MZI. However, in the free-carrier dispersion effect, the injected free carrier affects not only the real part of the refractive index, but also the imaginary part of the index, i.e., it will cause the attenuation of the light while it modulates the phase. As a result, optical switches of the MZI type can not reach a very high extinction ratio due to the fact that the two light beams from two arms of the MZI will have the different power level when they interfere.